In contrast to the often elaborate analytical procedures and unwieldy, expensive equipment sometimes required in clinical diagnoses in the recent past, advances in medical research have led to the development both of simpler procedures and equipment. Whereas previously employed techniques involved multiple steps and, in some cases, required both a high degree of dexterity and familiarity with the test procedures, kits and devices are now available which in some cases allow one with only limited laboratory experience to quickly and accurately perform diagnostic tests. Many of the techniques and devices provide an optical indication of a positive test, such as a color change, resulting from the reaction of (1) a substance being analyzed for (analyte), and (2) a test reagent or reagents. The analyte and/or test reagent may be a simple chemical substance or a more complex macromolecular substance from a biological fluid. The optical indication of the presence of the analyte may take place in solution, or on a solid surface, such as the interior surface of a glass or plastic tube, the well of a spot or a multiwell plate, or on a porous material such as a filter paper or membrane. Because of the typical white background of a porous material, a preference has existed for such materials in some instances and has also been incorporated in the form of porous membranes or matrices in test devices used alone or in specific test kits.
Typically, such devices locate the porous membrane in the bottom of a well adapted to receive the analyte-containing liquid. With the membrane treated with a test reagent or reagents prior or subsequent to introduction of the analyte-containing liquid to the well, the liquid passes through the membrane whereby sufficient analyte is specifically or nonspecifically adsorbed or retained on or within the membrane to undergo an optically indicating reaction with the test reagent(s). Typically, the liquid passes through the membrane due to gravitational forces, possibly assisted by a wicking or capillary action of an adsorbent material placed in contact with a surface of the porous membrane below or on the downstream side of the membrane.
Such means for removing liquid from a liquid analyte sample while retaining sufficient analyte on or within the membrane to perform tests on the analyte suffers from certain shortcomings. In some situations, the porous member exhibits a relatively low affinity for the analyte and is of a highly porous nature. Thus, the liquid sample containing the analyte passes through the porous member so rapidly that insufficient analyte is retained on or within the member to perform diagnostic tests. In other instances, passage of liquid through the member is too slow to make rapid diagnostic tests feasible. This has been the motivation for designing devices to increase the flow of analyte-containing fluid through the porous member. Although an improvement over gravity-induced passage of liquid, devices which employ an absorbent member in contact with the porous member to increase flow, are still too slow in many instances to provide satisfactory drainage from a porous member. This is particularly the case when the analyte-containing liquid is viscous or when, because of the analyte being tested for, a porous member with very small pores is required. The use of such devices thus limits the types of samples and analytes which may be tested. In some situations, although such devices provide adequate drainage rates, the flow through the porous member is uneven, due largely to air blockage below the porous member or non-linear capillary wicking into the adsorbent below.
There are some devices which provide adequate drainage rates by being connected to a source of differential pressure. Thus, variants of a suction flask and filter funnel have been used to obtain more rapid drainage or flow rates and, concomitantly, more rapid test results. However, such devices require differential pressure sources, such as a vacuum pump or an aspirator line. Such means for providing a differential pressure across the porous member, while effective in improving drainage rates or flow through the porous member, are not always available and, when available, are sometimes difficult to adjust or control the vacuum (differential pressure) generated.
For the reasons discussed above, there is a need for a device which provides not only a suitable porous member to permit retention of an analyte and test reagents but which also is operatively associated with a means for providing a rapid, but controlled, passage of liquid through the porous member. Preferably, the device should contain the means for providing a rapid and uniform passage of liquid through the porous member in a single, self-contained unit.